The ocean can extend in some places down to depths of several miles, or kilometers. However, most of the action takes place in the first couple hundred feet, or meters. Below this depth it is too dark, cold and murky for much to happen. Across the entire planet, the Earth’s oceans are believed to be basically the same below the first couple hundred feet, or meters.

Above this depth, where sunlight reaches, the traits found change dramatically from one location to another. Characteristics such as temperature, turbulence, and salinity (the amount of salt) can be very different from location to location.